The invention generally relates to the field of ignition spark timing circuits for internal combustion engines. More particularly, the present invention relates to the field of electronic ignition timing advance circuits in which a speed dependent advance angle for a spark timing ignition signal is produced and an advance adjustment for this spark timing signal is provided in accordance with a variable engine condition. In a specific embodiment of the present invention, the variable engine condition corresponds to the magnitude of the vacuum pressure produced by the internal combustion engine.
In standard internal combustion engines, a spark is used to ignite a fuel and air mixture in a cylinder. The proper timing of the occurence of this ignition spark with respect to the compression cycle of the cylinder has generally been difficult to implement in prior art systems. This is because the spark timing which is required for the efficient operation of the engine is generally a complex function of a great number of different engine variables. Primarily, ignition spark timing is a function of the speed of the engine and the engine load, the engine load commonly being sensed by the amount of vacuum pressure produced by the engine. The engine speed is commonly measured in terms of the angular rotational speed of the engine crankshaft in revolutions per minute. The "timing" of the spark ignition signal refers to the time occurrence of a spark timing pulse with respect to a predetermined angular position of the engine crankshaft, the crankshaft angular position corresponding to a specific point in the compression cycle of the cylinders. Thus the term "timing" as used in this specification refers to the time occurrence phase that exists between the occurrence of a spark timing pulse and a predetermined angular position of the engine crankshaft.
Prior art spark timing circuits have generally mechanically created a phase versus engine speed variation for the spark timing signal by using the centrifugal force created by the engine crankshaft rotation to produce a predetermined displacement of a cam which controls the amount of ignition advance. The term "advance" is generally used to describe the phase between the spark timing signal and a predetermined angular position of the engine crankshaft which corresponds to the TDC (top dead center) position of the piston in a cylinder. Typically, "advance" is stated in terms of an "advance angle" which corresponds to the phase difference between the spark timing signal and a reference angular position of the crankshaft given in degrees of crankshaft rotation.
A sensor is generally used to sense the vacuum pressure produced by the engine and this sensor is used to control the physical displacement of a cam which produces a predetermined amount of spark timing advance as a function of engine manifold pressure. Typically this vacuum advance is added to the speed dependent advance determined by the centrifugal force produced by the rotation of the engine crankshaft. This prior art system is generally a totally mechanical system which is extremely complex, difficult to adjust for a specific desired phase versus speed and vacuum characteristic and extremely costly since it includes a large number of mechanical parts which must be precisely manufactured. In addition, the mechanical nature of this system results in an inherent unreliability due to the physical wearing out of the mechanical parts.
Prior art electronic timing circuits do exist and have overcome many of the disadvantages of the mechanical spark timing circuits. Generally, these circuits have been unable to accurately and simply produce a spark timing signal in which a predetermined speed versus phase characteristic is produced which can be readily adjusted in accordance with a variable engine condition, such as engine manifold pressure. This problem is compounded when the desired phase versus speed variation comprises a plurality of different linear variation portions each operative over an associated predetermined range of engine speed.
Generally, it is desirable to produce spark timing signal having a characteristic which has different constant linear rates of change for its advance for a plurality of different speed ranges. Thus a phase versus engine speed characteristic which has the appearance of a piecewise linear graph comprising several line segments is the desired advance characteristic for a spark timing signal. Adjusting this desired speed dependent characteristic by a fixed amount of advance results in complex and costly circuitry according to the teaching of the prior art.